Signal Transduction and Targeted Therapy www.nature.com/sigtrans REVIEW ARTICLE OPEN Ribosomal proteins and human diseases: molecular mechanisms and targeted therapy ✉ Jian Kang1,2, Natalie Brajanovski1, Keefe T. Chan1,2, Jiachen Xuan1,2, Richard B. Pearson1,2,3,4 and Elaine Sanij1,2,5,6 Ribosome biogenesis and protein synthesis are fundamental rate-limiting steps for cell growth and proliferation. The ribosomal proteins (RPs), comprising the structural parts of the ribosome, are essential for ribosome assembly and function. In addition to their canonical ribosomal functions, multiple RPs have extra-ribosomal functions including activation of p53-dependent or p53- independent pathways in response to stress, resulting in cell cycle arrest and apoptosis. Defects in ribosome biogenesis, translation, and the functions of individual RPs, including mutations in RPs have been linked to a diverse range of human congenital disorders termed ribosomopathies. Ribosomopathies are characterized by tissue-specific phenotypic abnormalities and higher cancer risk later in life. Recent discoveries of somatic mutations in RPs in multiple tumor types reinforce the connections between ribosomal defects and cancer. In this article, we review the most recent advances in understanding the molecular consequences of RP mutations and ribosomal defects in ribosomopathies and cancer. We particularly discuss the molecular basis of the transition from hypo- to hyper-proliferation in ribosomopathies with elevated cancer risk, a paradox termed “Dameshek’s riddle.” Furthermore, we review the current treatments for ribosomopathies and prospective therapies targeting ribosomal defects. We also highlight recent advances in ribosome stress-based cancer therapeutics. Importantly, insights into the mechanisms of resistance to therapies targeting ribosome biogenesis bring new perspectives into the molecular basis of cancer susceptibility in ribosomopathies and new 1234567890();,: clinical implications for cancer therapy. Signal Transduction and Targeted Therapy (2021) 6:323; https://doi.org/10.1038/s41392-021-00728-8 INTRODUCTION together modulate protein synthesis and thereby cell growth rate Cell growth and proliferation are two distinct but coupled and proliferation.10,13–16 biological processes that are directly dependent on the tight Defects in ribosome biogenesis and function account for the coordination of protein synthesis and metabolic activity. These pathogenesis of a heterogeneous group of diseases called biological capabilities are essential characteristics that enable ribosomopathies. Ribosomopathies are generally defined as cancer cells to sustain uncontrolled proliferation.1 The biosynth- diseases caused by mutations in RPs or factors associated with esis of ribosomes, the molecular machines that translate Pol I transcription and rRNA processing, resulting in the disruption messenger RNA (mRNA) into proteins,2 is a fundamental of ribosome production or assembly.17–21 There is a growing list of biological process that is intimately linked to cell growth and diseases that have been classified as ribosomopathies; however, proliferation and is considered to be one of the most energy- the extent to which defects in ribosome synthesis contribute to consuming processes in proliferating mammalian cells.3–6 clinical phenotypes remains to be defined. In Treacher Collins Ribosome biogenesis is a highly dynamic and coordinated Syndrome, Diamond–Blackfan anemia and Shwachman-Diamond process, in which ribosomal RNA (rRNA) is synthesized, modified syndrome, the defects in ribosome synthesis have been causally and assembled with RPs to form mature ribosomes. Ribosome linked to disease pathotypes. However, in other putative biogenesis takes place within specialized subnuclear compart- ribosomopathies such as X-linked-dyskeratosis congenita and ments known as the nucleoli7,8 and depends on the coordinated cartilage–hair hypoplasia–anauxetic dysplasia, Blooms and Werner regulation of the three DNA-dependent RNA polymerases (Pol I, syndrome and cohesinopathies, it is likely that stress responses Pol II, and Pol III), as well as the involvement of a plethora of associated with ribosomal defects and/or altered mRNA transla- transcription factors, small nucleolar RNAs (snoRNAs) and non-RPs tion contribute to a component of their disease phenotype and that cooperatively promote the transcription, modification and impact on disease severity.20 Ribosomopathies are characterized processing of rRNAs, synthesis of RPs and ribosome assembly2,9–12 by tissue-specific phenotypic abnormalities. For example, the (Fig. 1a). This process is exquisitely regulated by multiple signaling hematopoietic system is frequently affected, which is thought to pathways in response to growth factors, energy and nutrients that be due to tissue-specific sensitivity to p53 pathway activation in response to ribosome stress.22 Ribosomopathy patients also have 1Division of Cancer Research, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia; 2Sir Peter MacCallum Department of Oncology, University of Melbourne, Melbourne, VIC, Australia; 3Department of Biochemistry and Molecular Biology, Monash University, Clayton, VIC, Australia; 4Department of Biochemistry and Molecular Biology, University of Melbourne, Melbourne, VIC, Australia; 5Department of Clinical Pathology, University of Melbourne, Melbourne, VIC, Australia and 6St. Vincent’s Institute of Medical Research, Fitzroy, VIC, Australia Correspondence: Elaine Sanij ([email protected]) These authors contributed equally: Jian Kang, Natalie Brajanovski Received: 20 February 2021 Revised: 12 July 2021 Accepted: 30 July 2021 © The Author(s) 2021 Ribosomal proteins and human diseases: molecular mechanisms and targeted. Kang et al. 2 Fig. 1 A schematic representation of ribosome biogenesis in mammalian cells. a Ribosome biogenesis is a tightly coordinated process involving all three RNA polymerases (Pol I, Pol II, and Pol III). RNA Pol I transcribes the ribosomal RNA (rRNA) genes (rDNA) to produce the 47S precursor rRNA (47S pre-rRNA) transcript in the nucleolus. Pol I transcription initiation involves binding of the upstream binding factor (UBF) to the core promoter region (core) and upstream control element (UCE) of the rDNA promoters and facilitating the recruitment and binding of the selectivity factor 1 (SL-1) complex. SL-1 is composed of the TATA-box-binding protein (TBP) and five Pol I-specific TATA-box-associated factors (TAFs). This complex in turn recruits the Pol I-specific initiation factor RRN3, which associates with DNA topoisomerase IIα (TOPIIα) and Pol I to complete assembly of a transcriptionally-competent Pol I complex. Following transcription, the 47S pre-rRNA is subsequently cleaved and processed into the mature 18S, 5.8S, and 28S rRNA species. These molecules are then assembled along with ribosomal proteins and the 5S rRNA produced by Pol II and III, respectively, to form the major catalytic and architectural components of the small (40S) and the large (60S) ribosomal subunits. Once assembled, ribosomal complexes are exported from the nucleolus to the cytoplasm, where they form the mature (80S) ribosome required to initiate mRNA translation and thus protein synthesis. b A diverse range of anticancer drugs target ribosome biogenesis by inhibiting Pol I transcription and/or pre-rRNA processing a higher risk of developing cancer.23 The paradoxical transition (e.g., oxaliplatin, cisplatin, actinomycin D and 5-fluorouracil (5-FU)) from early symptoms due to hypo-proliferative phenotypes to an have been discovered to act through distinct mechanisms of action increased cancer risk later in life, was first reported by Dameshek that include inhibition of rRNA synthesis, rRNA processing or in 1967 and referred to as Dameshek’s riddle.24 The mechanisms ribosome biogenesis34–38 (Fig. 1b). The development of a number by which genetically compromised ribosome biogenesis leads to of less genotoxic drugs that selectively target Pol I transcription and hyper-proliferative cancer phenotypes in patients with ribosomo- ribosome biogenesis has established a new paradigm for cancer pathies remain a mystery. therapy.36,39–42 These compounds are active against subsets of The recent discoveries of somatic mutations in RP genes in tumors but their therapeutic response can be more potent in hematological cancers (such as T-cell acute lymphoblastic cancers with deregulated ribosome biogenesis.43–45 In this review, leukemia (ALL), chronic lymphocytic leukemia (CLL) and multi- we discuss the rationale for targeting ribosome biogenesis as a ple myeloma) and solid tumors (such as breast cancer and treatment strategy to combat cancer and the current under- melanoma) emphasize that defects in ribosome biogenesis standing of the therapeutic potential of the first-in-class inhibitor of could potentially promote oncogenic transformation.23 In fact, Pol I transcription CX-5461. Importantly, recent studies of the key deletions in RP genes are common events across human molecular mechanisms of acquired resistance to selective inhibition cancers, particularly in concert with TP53 mutations. This is in of ribosome biogenesis provide a new conceptual framework to contrast to mutation in RPs (without concurrent TP53 mutation) expand on the understanding that specific rewiring of translation in that are primarily associated with ribosomopathies.25 Here, we response to chronic ribosome stress promotes cancer progression. review the current knowledge and new perspectives with We highlight